Reorder field in the block

Miners should choose a transaction from the network based on its reward,
not as part of the mining variability.
That's why it's a bad idea to place the transaction as last block element.

Reorder the fields to make less room for harmful mining optimization.

docs/protocol-v0.md

@@ -168,10 +168,10 @@ The node should answer to a "Mining task request" with a "Mining task response"
 | protocol version = 0 (BE) | 2 |
 | capable version = 0 (BE) | 2 |
 | type = 8 (BE) | 1 |
-| timestamp (unix time in seconds, BE) | 8 |
-| previous hash | 32 |
-| difficulty sum (BE) | 32 |
 | transaction (optional) | 148 |
+| previous hash | 32 |
+| timestamp (unix time in seconds, BE) | 8 |
+| difficulty sum (BE) | 32 |
 
 The node tells the miner "timestamp", "previous hash", "difficulty sum" and "transaction" for the new block.